Process for automatically regulating the reduction of the iron and titanium values in a digestion liquor



Dec. 30. 1969 H. STEINHAUSEN 3, 86 847 PROCESS FOR AUTOMATICALLY REGULATING THE REDUCTION OF THE IRON AND TITANIUM VALUES IN A DIGESTION LIQUOR Filed Sept. 15, 1967 v INVENTOR.

. I Helmut Steinhqusen W M I AGENT United States Patent Int. Cl. G01n 33/24, 27/32, 31/00; C01g 23/06 US. Cl. 23-117 4 Claims ABSTRACT OF THE DISCLOSURE The invention covers a process for the continuous and fully automatic reduction of the iron and titanium values in digestion of liquors produced from titanium ores, titanium ore concentrates and titanium slags with the aid of a regulating device which controls the rate of flow of the digestion liquor through a reductor filled with a reducing agent by measuring the oxidation-reduction potential of the liquor which is dependent upon the amount of tri-valent titanium ions formed during the reduction.

BACKGROUND OF THE INVENTION It is known in the art that in the so-called sulfate process for the manufacture of titanium dioxide pigments by digestion of titanium ores and titanium concentrates by means of sulfuric acid and subsequent solution of the digestion cake, the titanium sulfate solutions obtained are reduced by adding metallic iron, e.g. scrap iron (Barksdale: Titanium, 2nd edition, New York 1966, pp. .245- 246). In this the Fe ions are first transformed into R ions since the salts of divalent iron do not hydrolyze as easily as the salt of the trivalent iron, i.e. they are not hydrolytically decomposed under the conditions of the subsequent thermal hydrolysis of the titanium salts and cannot contaminate the titanium hydrolyzate.

In order to be certain that during the preparation of the digestion liquor for hydrolysis the iron values remain in the ferrous state and do not oxidize, it is customary (U.S. Patent No. 1,333,849) to permit the reduction to proceed far enough so that the titanium solution contains a small amount of trivalent titanium. In this it is essential that the content or trivalent titanium be within a definite range. A reduction that has gone too far gives a poor yield in the hydrolysis, i.e. it leads to TiO losses. Too slight a reduction, on the other hand, makes it uncertain whether sufficient trivalent titanium is always present up to the washing of the hydrolyzate to prevent the iron values from oxidizing. The control of the Ti value must therefore always be carried out; it is mostly carried out by titration with an iron (III) sulfate solution of known content using thiocyanate solution as indicator. This analytical method of determination requires quite some time so that a rapid and/or continuous reduction is not very well controllable in this manner.

SUMMARY OF THE INVENTION It has been found that the course of the reduction can be controlled very well and the amount of Ti reduced to Ti can be measured exactly and rapidly if the oxidation-reduction potential is measured directly in the digestion solution and that the change of the potential, occurring in the reduction process, is used for regulating the amount of throughput or the rate of flow through a reductor filled with a reducing agent, preferably scrap iron. When the potential of a digestion liquor as a function of its content of Fe ions or Ti ions is measured with ice the aid of a platinum-calomel electrode measuring combination, a jump in potential rapidly takes place between about 4 g.p.l. Fe and about 5 g.p.l. Ti

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is presented to show the rapid jump in potential which takes place in an iron sulfate-titanium sulfate solution when all of the iron values have been reduced to the ferrous state and a small amount of trivalent titanium is formed.

FIGURE 2 represents a schematic drawing of apparatus for carrying out the continuous and automatic reduction of the iron sulfate-titanium sulfate solution to obtain the desirable degree of reduction.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the manufacture of T iO pigments using the sulfate process, a titaniferous material is digested in concentrated sulfuric acid to form an iron sulfate-titanium sulfate solution. The iron values in solution are present both as ferric sulfate and ferrous sulfate and the titanium values are present as tetravalent titanium sulfate. Before hydrolyzing the solution it is necessry to reduce the ferric iron values to the ferrous state so that the titanium hydrates formed will not be contaminated with any ferric salts. As previously stated this is usually accomplished by adding scrap iron to the soltuion which reduces the ferric iron values to the ferrous state and in order to insure that all of the iron remains in the ferrous state, a small portion of the titanium values are reduced to the trivalent form i.e. a few grams per liter. As long as a slight amount of trivalent titanium is present in the solution, no ferric iron will reform.

In the instant invention the degree of reduction is measured by the oxidation-reduction potential of the solution. The amount of Ti+ reduced to Ti+ is measured exactly and rapidly by a change in the potential. The dependence of the oxidation-reduction potential T i +/Ti is surprisingly large in the range up to about 5 g.p.l. Ti calculated as TiO so that it is easily possible to coordinate each measured potential with a definite content of Ti ions. The potential curve must, of course, be determined separately for each digestion liquor produced according to a particular digestion process from a particular titaniferous material. However with each identical starting material and identical digestion procedure, the same standardizing curve may be used each time. It was a surprising fact that at equal digestion procedures and at equal Ti content the small, unavoidable variations of the compositions of the solutions and of the temperature necessary for reduction have practically no appreciable effect on the oxidation-reduction potential, as shown by a. comparison of the measuring values found at 65 C. and 45 C., recorded in FIGURE 1.

Thus, it is possible to calibrate the scale of the potentiometer used for measuring directly the concentration of the Ti ions, and therefore, to read immediately the Ti concentration, corresponding to a definite potential value.

This procedure not only renders the determinations of the Ti content faster and more exact, the analytically determined value is at the same time formed into an electrical measuring value which, in the case of continuous reduction in a reductor, permits automatic control of the reaction.

Such an automatic control of the continuous reduction may be carried out, for example, by means of the apparatus described in FIG. 2.

The apparatus consists of a bufl er and mixing tank 2 fitted with a stirrer. The unreduced digestion liquor is fed into this mixing tank through the inlet 1. The amount of liquor fed may be regulated by valve V The sufi'iciently reduced liquor is transferred from the mixing tank into the clarifying tank through outlet 11, whereby the effluent amount may also be controlled by valve V The liquor is pumped through pipe 3 from the mixing tank 2 into the reductor 5 by means of pump 4 for reduction; the reductor has a vent 6. The amount flowing through is regulated by valve V From there the reduced solution flows through pipe 7 again into the mixing tank 2 into which the electrodes 8 connected with the potentiometer 9 are immersed directly in front of the outlet. The potentiometer is connected with the regulator 10 which controls the regulating valves V V and V In order to carry out the process according to the invention the unreduced digestion liquor coming from the digestion tank is brought to the temperature necessary for reduction and collected in the mixing tank. From there it is pumped in portions into the reductor which is filled with loose scrap iron, where it emanates from a ring-shaped pipe with many bore holes like a sprinkler, and flows over the scrap iron. The reductor, which is expediently suspended in an elevated frame, can be refilled with scrap iron from above. The reduced solution is recycled into the mixing tank again through the bottom outlet of the reductor and mixed there with the remaining solution. The potential of this solution measured by the oxidation-reduction electrode measuring device is indicated by the potentiometer and effects the control of the regulating valve V via the regulator, whereby the ratio of flow through the reductor to the total amount of flow through the mixing tank is regulated corresponding to the content of Ti ions. The regulator causes, furthermore, closing of the regulating valve V in the inlet of the mixing tank when the Ti ion content has been decreased below a certain minimum value, and a closing of the regulating valve V when the Ti ion content has risen above a certain value. Since in this case unreduced liquor is added without reduced liquor being removed, it may be necessary to close the influx to the tank by means of a floating hook-up at a certain level of filling.

According to a somewhat modified arrangement the electrode measuring combination may be immersed, instead of into the mixing tank directly before the outlet, into the outlet itself even before the regulating valve V The flow through the reductor may be carried out with potentiometric control in other form also in the main circuit and/ or shunt circuit.

The redox electrode measuring combination may be the combination of a platinum electrode immersed into the measuring solution and a reference electrode which is separated from the measuring solution by a suitable salt bridge; as reference electrode, for example, a calomel electrode or another reference electrode which operates reversibly at elevated temperature is applicable. Reference electrodes need some attention in continuous operation. In order to avoid the disadvantages connected with this, it is advantageous in certain cases to use the quite constant oxidation-reduction potential of the unreduced starting solution as reference potential, i.e. to employ two platinum electrodes of which one is immersed in the unreduced digestion liquor and the other inthe reduced digestion liquor which is, as the case may be, connected with the unreduced liquor via a diaphragm. In this manner the reduction may be carried out up to a definite potential difference which must be determined empirically.

It is also possible to carry out the reduction electrolytically or with reducing agents other than metallic iron.

The novel process for the control of the reduction has the advantage, compared with prior art procedures in that;

(1) It takes a completely independent course,

(2) It leads to a constant Ti ion content so that the desired value of the Ti ion content may be lower altogether which fact produces lower losses in the hydrolysis.

(3) It permits reduction as rapidly as desired since no additional time is required for the determination of the degree of reduction. This is particularly advantageous when digestion liquors are processed which are less stable towards hydrolysis and which must be quickly cooled back to lower temperatures from the higher temperatures needed, as the case may be, for the reduction.

The process according to the invention is not only limited to sulfuric acid digestion liquors. It may, in the same manner, be applied to hydrochloric acid solutions.

I claim:

1. In the method of digesting an iron containing titanium material selected from the group consisting of titanium ores, titanium concentrates and titanium slags wherein said titanium material is reacted with an acid and subsequently expose to a reducing agent to reduce all iron values in the liquors formed in the digestion and wherein the exposure to said reducing agent is regulated to provide a controlled amount of trivalent titanium in said liquors, said regulation being performed by measuring the amount of trivalent titanium in the liquors and then regulating the exposure of said liquors to said reducing agent, the improvement wherein said measuring is performed by inserting a pair of electrodes in said liquor and measuring the redox potential therein and subsequently converting said redox potential to an impulse signal regulating the exposure of said liquor to said reducing agent.

2. The method of claim 1 wherein the reducing agent is iron.

3. The method of claim 1 wherein the electrodes are platinum.

4. The method of claim 1 wherein one electrode is platinum and the other electrode is a calomel electrode.

References Cited UNITED STATES PATENTS 1,333,849 3/1920 Olsen et al. 204-96 1,916,236 7/1933 Ryan 23-117 XR 3,199,948 8/1965 Clerbois et al. 137-93 XR 3,218,131 11/1965 Grose 23-117 XR 3,368,870 2/1968 Soloducha 23-117 XR JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner 

